The present invention pertains to the field of chemical vapor deposition of tin, titanium, and zinc oxides onto glass and other substrates, and to tin, titanium and zinc compounds useful therefor.
Tin oxide, titanium oxide, and zinc oxide films have been applied to glass substrates for a variety of purposes, notably as infrared absorbing coatings, and as transparent conductive electrodes for use in such devices as photovoltaic cells and dimmable mirror assemblies. In such applications, the physical, electrical, and optical characteristics of the metal oxide films are often critical. Among these characteristics are coating thickness, uniformity, smoothness, electrical conductivity or resistivity, spectral transmission, and optical clarity.
Tin oxide films have become commonplace. However, most applications involving SnO2 films are relatively high product volume applications which require that the chemical vapor deposition (CVD) precursor tin compounds be inexpensive, and preferably stable. For titanium oxide and zinc oxide films, similar considerations apply. Thus, despite their ability to provide high quality films, numerous organometallic compounds of tin, titanium, and zinc cannot be used, either because they are too expensive or because of the dangers associated with use of highly flammable and toxic compounds in a commercial industrial setting.
Tin oxide and titanium oxide films have been prepared by contacting separate carrier gas streams containing tin tetrachloride or titanium tetrachloride and either water or an oxygen-containing compound which reacts with the metal halide at elevated temperatures. The streams are contacted with each other physically close to the substrate onto which the coating is to be deposited. These methods have not proven to be totally satisfactory, even though the tin halide/water reaction is widely used. Tin tetrachloride and titanium tetrachloride are volatile and highly reactive. However, the greatest difficulty with the use of metal halide/water to form metal oxide films is the formation of a fine dust or powder of tin oxide particulates in addition to the coherent metal oxide coating. The presence of these oxide particles necessitates the frequent shut down and cleaning of the coating apparatus.
For zinc oxide coating, dialkylzinc compounds have been used in conjunction with an oxidant such as water or an oxygen-containing organic compound, as disclosed in Vijayakumar et al. U.S. Pat. No. 4,751,149. Dialkylzinc compounds are highly reactive and spontaneously flammable in air. They react explosively with water. Thus, their use in large scale coating is highly problematic from a safety standpoint. Moreover, employing two separate reagent streams and allowing them to combine and react adjacent the substrate again produces particulates as well as the desired coherent film.
The foregoing methods have the additional disadvantage that the compositional nature of the film may change due to factors such as carrier gas flow, pressure, and temperature, as well as the concentration of the reactive ingredient in the carrier gas streams. With a constant ratio of reactive ingredients, films with relatively constant stoichiometry but varying smoothness and thickness may result due to variations in the foregoing carrier gas parameters. If the reactant ratios also vary, films of different stoichiometry, electrical, and optical properties will result. Examples of tin and titanium oxide films with varying properties prepared by different ratios of tin or titanium tetrachloride and organic carboxylic acid esters in separate gas streams is given in PCT published application WO 98/06675.
Use of single component tin oxide, titanium oxide, or zinc oxide CVD precursor systems requiring but one supply stream has thus far not met with success, either because of the cost of the organometallic precursors, or because the desired film thickness and uniformity cannot be achieved. In non-critical applications such as the surface modification of tempered hot glass, solutions of tin and titanium compounds dissolved in organic solvent have been used, the organic solvent also functioning as a potential oxidizing agent. Examples of tin and titanium oxide coatings to strengthen molded glassware, and formed by applying solutions of tin or titanium halide in excess organic esters of acetic, propionic, or butyric acids are given in Great Britain Patents GB 1,187,784 (1967; tin) and GB 1,187,783 (1967; titanium). The use of excess organic solvent renders these processes less ecologically desirable today. Moreover, application of the solutions by spraying renders precise control of film physical, chemical, and electrooptical parameters virtually impossible, and encourages carbon contamination. Thus, such coatings cannot be used for products such as dimmable mirrors and photovoltaic cells.
It has now been surprisingly discovered that novel ligated compounds of tin, titanium, and zinc can be used to prepare uniform, high quality metal oxide coatings on glass and other substrates without the use of separate reactant streams and without application in the form of a solution. These tin, titanium, and zinc compounds have exhibited high relative stability as compared to precursors such as tin tetrachloride, titanium tetrachloride and dialkylzinc.
The tin and zinc compounds of the present invention have the formula MXnL2 where M is Sn or Zn and L is a C1-4 alkylformate, preferably ethylformate; X is Cl and n is 4 when M is Sn; and X is C1-4 lower alkyl and n is 2, when M is Zn, with the proviso that when M is Sn, the ethylformate ligands are preferably positioned cis to each other. The preferred chemical vapor deposition compounds are 
where R is xe2x80x94CH3 or xe2x80x94CH2CH3. As will be discussed infra, these compounds are well characterized, easily synthesized, and form excellent coatings. The zinc compounds are far more stable then dialkylzinc compounds previously used to produce zinc oxide coatings; for example, rather than react explosively with water, they slowly react, liberating alkane gas in the process. The compounds are low melting solids or liquids which can easily accept dopant compounds, allowing a means of providing a stable, constant concentration of dopant atoms to the coating process rather than employ a yet further stream of dopant compound. Examples of C1-4 alkyl groups in the alkyl formates include methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, and t-butyl alkyl groups.
Thus, in general, the tin and zinc compounds have the formula:
MXnL2
where
M is Sn or Zn;
L is selected from the group consisting of methylformate, ethylformate, n-propylformate, i-propyl formate, n-butylformate, i-butylformate, t-butylformate, and mixtures thereof;
X is Cl and n is 4 when M is Sn; and
X is R and n is 2 when M is Zn,
where
R is C1-8 lower alkyl or C2-8 lower alkenyl; and
when M is Sn, the ethyl formate ligands are preferably positioned cis to each other.
The preferred tin compound may be prepared by contacting tin tetrachloride in the gas phase with gaseous ethyl formate, preferably in stoichiometric ratio (1:2, respectively). The contact is preferably performed at relatively low temperatures, i.e., at room temperature, and in an inert atmosphere, preferably a dry nitrogen atmosphere. Upon contact of the gas streams, large white crystals of the desired product immediately form. Alternatively, the reactants may be dissolved in a non-polar aprotic solvent such as hexane. The white crystalline product may be recovered by filtration. The compound is characterized by a melting point of 52-53xc2x0 C.; 1H NMR (CDCl3, ppm) 1.33 (t, 3H, CH3), 4.31 (q, 2H, CH2), 8.18 (S, 1H, CH); 13C NMR (CDCl3, ppm) xe2x88x920.02, 14.0, 61.5; 119Sn NMR xe2x88x92823. The IR spectrum showed notable absorbance at 1717cmxe2x88x921 (KBr disk) and 1616 cmxe2x88x921 (gas phase). X-ray crystallography indicates a molecule with ethyl formate ligands occupying cis positions relative to each other.
The obtained compound is surprisingly different from the tin chloride/ethyl formate complex of the same empirical formula prepared by dissolving tin tetrachloride and ethylformate in excess ethylformate, as reported by Paul, et al., Ind. J. Chem. 7, 377-8880 (April 1969). The compound prepared by Paul et al. is apparently a different compound, as evidenced by a quite different melting point of 63xc2x0 C. No structural assignment (NMR; IR; Crystallography) was reported, and thus the different melting point may be reflective of an addition reaction across the Oxe2x95x90C bond of the formate ester rather than the complexes of the present invention. However, the configuration does not appear to be important in coating processes.
The zinc CVD precursors are preferably prepared by complexing the corresponding dialkylzinc compound with ethylformate in non-reactive solvent. Suitable non-reactive solvents include aromatic and aliphatic hydrocarbons, preferably hexane. The reaction may take place at room temperature or at higher temperatures, but advantageously is performed at lower temperatures such as 0xc2x0 C. The ethylformate is added gradually to the solution of dialkylzinc in solvent. The reaction is exothermic, and the resultant product is obtained as a clear homogenous solution in solvent. The solvent may be separated from the product by distillation if desired.
Titanium halide: alkylformate complexes have also been characterized, and found suitable for preparation of titanium oxide coatings on heated substrates. Both 1:1 and 2:1 alkylformate: titanium tetrachloride compounds exist. These complexes have the formula TimX2mL2, where m is 1 or 2 and X and L have the meanings set forth previously. Coatings prepared to date from these precursors have required addition of air or oxygen during the coating process to eliminate carbon contamination, when so desired. As with the tin and zinc coatings, the titanium coatings may also be doped. Particularly useful dopants include a wide variety of metal halide: alkylformate complexes. However, metal halides, etc., can be used as well.
In use, the CVD precursors are directed to a hot substrate, i.e., glass, in a single stream. By the term xe2x80x9csingle streamxe2x80x9d is meant that separate streams of reactants are avoided. Multiple xe2x80x9csingle streamsxe2x80x9d may be employed to form coatings, for example on large area substrates, or to form multiple coatings with increased total coating thickness or different dopant concentration. The substrate is coated at any convenient pressure that allows the CVD precursor to volatilize, preferably at normal atmospheric pressure or below, i.e., water jet pump vacuum. The substrate temperature is preferably maintained at 400xc2x0 C. to 700xc2x0 C., more preferably 450xc2x0 C. to 600xc2x0 C., and most preferably about 500-550xc2x0 C.
Dopant compounds such as group 13 or group 15 halides may be added to the CVD precursor compounds or their solutions to provide any desired level of dopant concentration. Dopants from other groups of elements, i.e., copper, silver, gold, among others, may be used as well. In general, many groups of elements may serve as dopants, particularly groups 4, 12, 13, 14, and 15 of the periodic table of the elements. The electrical resistivity may be varied from the high megohm range to values below 100xcexa9. Dopant compounds other than halides may also be used, such as ethylformate complexes of boron or aluminum alkyls, etc. An example is the ethylformate complex of trimethylaluminum, as described hereafter.
Deposition rates achieved by the subject invention CVD precursor compounds are high, and film quality excellent. While the CVD compounds are preferably contacted with the hot substrate in the gas phase, other application methods may be used for less demanding applications.
Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified.